The statements in this section merely provide background information related to the present disclosure and may not constitute prior art.
A powertrain utilizing a rotational input torque through an input shaft to drive an output shaft is known to utilize a transmission to change gear states. Such a rotational input torque can come from a number of devices, individually or in cooperation, including an internal combustion engine, an electric machine, or other similar devices capable of providing the input torque. Such a powertrain can exist within a vehicle or in a non-vehicle application.
Operation of a powertrain requires management of numerous torque bearing shafts or connections among the above mentioned engine, electrical machines, input shaft, output shaft, and driveline. Shifting between gear states in an automatic transmission may require that at least one clutch be loaded or unloaded. In an exemplary clutch utilizing hydraulic pressure in combination with pistons selectively applying force based upon the hydraulic pressure, a shift in gear states or a gear shift requires that one clutch be unloaded, permitting two shafts that were previously coupled to spin freely of each other, and subsequently that another clutch be loaded, coupling two shafts that were formerly free to spin relative to one another. The coupling of the various shafts defines the resulting relationship between the input shaft and the output shaft. In one exemplary transmission, the various shafts control operation of a planetary gear set through which the necessary mechanical transformations of the transmission are made.
A control system must be able to accurately command the necessary transitions within the transmission to control desired gear shifts. Performing the transitions in a disorderly or non-sequential manner may cause undesirable performance, including adverse impacts to drivability or passenger perceptible changes to the operation of the vehicle.
Clutches are devices well known in the art for engaging and disengaging shafts including the management of rotational velocity and torque differences between the shafts. Clutches are known in a variety of designs and control methods. One known type of clutch is a mechanical clutch operating by separating or joining two connective surfaces (e.g. clutch plates) operating, when joined, to apply frictional torque to each other. One control method for operating such a mechanical clutch includes, as described above, applying a hydraulic control system implementing fluidic pressures transmitted through hydraulic lines to exert or release clamping force between the two connective surfaces. A gear shift frequently involves exhausting fluid from the piston of one clutch and filling fluid in another clutch. It will be appreciated that while orderly decoupling and coupling of the various shafts within the transmission is desired, it is also desired to accomplish the required transitions in as short a time period as possible. As a result, a filling of an on-coming clutch to be coupled can begin while an off-going clutch is still coupled, for example, with its process of exhausting the associated piston having just begun. Exemplary optimization of such a transition may include exhausting the off-going clutch to a point where normal forces between the associated clutch plates are reduced to zero just before normal forces between the associated plates of the on-coming clutch exceed zero.